The emergence of DWDM technology as a viable solution to bandwidth bottlenecks in current and future optical networks has generated a need for spectrally tunable optical components. In particular, tunable laser sources that can be deployed along with high speed, intensity modulator components are required. Several different technologies and approaches have been adopted in an effort to provide a low-cost, high performance, tunable laser sources that meet the specifications called for by today's network operators.
A number of tunable laser implementations make use of micro-electro-mechanical systems (MEMS) for their successful operation. These include ECLs and DFB arrays where a mechanical adjustment allows selection of the desired laser wavelength. The DFB array approach has particular attractions in that well-understood, reliable DFB laser technology is used, albeit in array form. In this implementation, up to 12 DFBs are fabricated on the same chip with a separation of around 10 μm between adjacent lasers. The center wavelength, at fixed temperature, is offset by approximately 3 nm between adjacent lasers and each laser can be temperature tuned to cover 3 nm. Thus, full C band tuning can be realized by selectively tuning the lasers within their local thermal range and switching to the adjacent laser as required.
However, the spatial offset of the individual lasers within the array makes efficient coupling to a single optical fiber difficult. A single lens can be used to collect and collimate the light from each of the DFB laser sources but the spatial offset results in a varying angle on the collimated beam. This leads to a spatial offset when the beams are focused onto a single optical fiber that is located in a fixed position in the focal plane of a second lens. Thus, efficient coupling can only be obtained for the DFB laser at the center of the array.
The above situation can be remedied by incorporating a beam steering element in the optical path between a collimating lens and a focus lens. Typically, this could be a MEMS mirror that can be electro-statically tuned via application of a control voltage. This approach suffers from vibration issues that are known to plague MEMS type implementations.
As a result, one object of this invention is to provide a low cost, low power consumption, multiplexing solution to coupling of an array of DFB lasers (spatially and spectrally offset). The described approach removes the need for a complicated MEMS beam steering mirror and reduces sensitivity to vibration effects, etc.